Granulocyte-macrophage Colony-stimulating Factor Receptor Research Articles

Critical Role of the GM-CSF Signaling Pathway in Macrophage Pro-Repair Activities

Objective: Macrophages play a critical role in intestinal wound repair. However, the molecular pathways that regulate macrophage wound repair activities remain poorly understood. The aim of this study was to evaluate the role of GM-CSF receptor signaling in the wound repair activities of macrophages. Methods: Murine macrophages were differentiated from bone marrow cells and human macrophages from monocytes isolated from peripheral blood mononuclear cells of Crohn's disease (CD) patients. In vitro models were used to study the repair activities of macrophages. Results: We provide evidence that GM-CSF receptor signaling is required for murine macrophages to promote epithelial repair. In addition, we demonstrate that the deficient repair properties of macrophages from CD patients with active disease can be recovered via GM-CSF therapy. Conclusion: Our data support a critical role of the GM-CSF signaling pathway in the pro-repair activities of mouse and human macrophages.

C-Abl Activates Janus Kinase 2 in Normal Hematopoietic Cells

Jak2 is involved in cytokine growth factor-stimulated signal transduction, but the mechanism of its activation is largely unknown. Here, we investigated Jak2 activation in a normal hematopoietic cell line, 32D mouse myeloid cells. The bimolecular fluorescence complementation studies showed that c-Abl formed a stable complex with Jak2 in live cells. Co-immunoprecipitation results showed that c-Abl bound to the βc chain of IL-3/IL-5/GM-CSF receptors. The kinase activities of both c-Abl and Jak2 were stimulated by IL-3 in 32D cells. Decreasing c-Abl protein expression in 32D cells by inducible shRNA decreased Jak2 activity and resulted in the failure of Jak2 activation in response to IL-3. Treatment of IL-3 and serum-starved 32D cells with 1 μM imatinib mysylate inhibited IL-3 stimulated kinase activities of both c-Abl and Jak2. In addition, the kinase-deficient Bcr-Abl mutant (p210K1172R) was defective for activation of Jak2 in 32D cells and impaired IL-3 independent growth, which was rescued by overexpression of c-Abl (+Abl). IL-3 efficiently inhibited apoptosis of 32Dp210K/R+Abl cells induced by imatinib mysylate but not Jak2 kinase inhibitor TG101209. In summary, our findings provide evidence that the kinase function of c-Abl and its C-terminal CT4 region is crucial for its interaction with Jak2 and its activation. c-Abl kinase activity induced by IL-3 is required for IL-3-stimulated Jak2 and Jak1 activation. Our findings reveal a novel regulatory role of c-Abl in Jak2 activation induced by IL-3 cytokine growth factor in 32D hematopoietic cells.

Engineered fusokine GIFT4 licenses the ability of B cells to trigger a tumoricidal T-cell response.

Engineered chimeric cytokines can generate gain-of-function activity in immune cells. Here, we report potent antitumor activity for a novel fusion cytokine generated by N-terminal coupling of GM-CSF to IL4, generating a fusokine termed GIFT4. B cells treated with GIFT4 clustered GM-CSF and IL4 receptors on the cell surface and displayed a pan-STAT hyperphosphorylation associated with acquisition of a distinct phenotype and function described to date. In C57BL/6J mice, administration of GIFT4 expanded endogenous B cells and suppressed the growth of B16F0 melanoma cells. Furthermore, B16F0 melanoma cells engineered to secrete GIFT4 were rejected immunologically in a B-cell-dependent manner. This effect was abolished when GIFT4-expressing B16F0 cells were implanted in B-cell-deficient mice, confirming a B-cell-dependent antitumor effect. Human GIFT4-licensed B cells primed cytotoxic T cells and specifically killed melanoma cells in vitro and in vivo. Taken together, our results demonstrated that GIFT4 could mediate expansion of B cells with potent antigen-specific effector function. GIFT4 may offer a novel immunotherapeutic tool and define a previously unrecognized potential for B cells in melanoma immunotherapy.

Preclinical evidence that PD1 blockade cooperates with cancer vaccine TEGVAX to elicit regression of established tumors.

Biomarker studies have shown that expression of the T-cell coregulatory ligand PDL1 on tumor cells correlates with clinical responsiveness to the PD1 antibody nivolumab. Here, we report the findings of a preclinical cancer vaccine study demonstrating vaccine-dependent PDL1 upregulation in the tumor microenvironment. We formulated an IFNγ-inducing cancer vaccine called TEGVAX that combined GM-CSF and multiple Toll-like receptor agonists to increase the number of activated dendritic cells. Treatment of established tumors with TEGVAX retarded tumor growth in a manner associated with enhanced systemic antitumor immunity. Unexpectedly, TEGVAX also upregulated PDL1 expression in the tumor microenvironment, possibly explaining why tumors were not eliminated completely. In support of this likelihood, PDL1 upregulation in this setting relied upon IFNγ-expressing tumor-infiltrating CD4(+) and CD8(+) T cells and administration of a PD1-blocking antibody with TEGVAX elicited complete regression of established tumors. Taken together, our findings provide a mechanistic rationale to combine IFNγ-inducing cancer vaccines with immune checkpoint blockade.

GM-CSF-licensed CD11b+ lung dendritic cells orchestrate Th2 immunity to Blomia tropicalis.

The Blomia tropicalis dust mite is prevalent in tropical and subtropical regions of the world. Although it is a leading cause of asthma, little is known how it induces allergy. Using a novel murine asthma model induced by intranasal exposure to B. tropicalis, we observed that a single intranasal sensitization to B. tropicalis extract induces strong Th2 priming in the lung draining lymph node. Resident CD11b(+) dendritic cells (DCs) preferentially transport Ag from the lung to the draining lymph node and are crucial for the initiation of Th2 CD4(+) T cell responses. As a consequence, mice selectively deficient in CD11b(+) DCs exhibited attenuated Th2 responses and more importantly did not develop any allergic inflammation. Conversely, mice deficient in CD103(+) DCs and CCR2-dependent monocyte-derived DCs exhibited similar allergic inflammation compared with their wild-type counterparts. We also show that CD11b(+) DCs constitutively express higher levels of GM-CSF receptor compared with CD103(+) DCs and are thus selectively licensed by lung epithelial-derived GM-CSF to induce Th2 immunity. Taken together, our study identifies GM-CSF-licensed CD11b(+) lung DCs as a key component for induction of Th2 responses and represents a potential target for therapeutic intervention in allergy.

Dual Mechanism of Interleukin-3 Receptor Blockade by an Anti-Cancer Antibody

Interleukin-3 (IL-3) is an activated T cell product that bridges innate and adaptive immunity and contributes to several immunopathologies. Here, we report the crystal structure of the IL-3 receptor α chain (IL3Rα) in complex with the anti-leukemia antibody CSL362 that reveals the N-terminal domain (NTD), a domain also present in the granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-5, and IL-13 receptors, adopting unique "open" and classical "closed" conformations. Although extensive mutational analyses of the NTD epitope of CSL362 show minor overlap with the IL-3 binding site, CSL362 only inhibits IL-3 binding to the closed conformation, indicating alternative mechanisms for blocking IL-3 signaling. Significantly, whereas "open-like" IL3Rα mutants can simultaneously bind IL-3 and CSL362, CSL362 still prevents the assembly of a higher-order IL-3 receptor-signaling complex. The discovery of open forms of cytokine receptors provides the framework for development of potent antibodies that can achieve a "double hit" cytokine receptor blockade.

Preclinical characterisation of the GM-CSF receptor as a therapeutic target in rheumatoid arthritis

ObjectivePrevious work has suggested that the granulocyte macrophage colony stimulating factor (GM-CSF)–GM-CSF receptor α axis (GM-CSFRα) may provide a new therapeutic target for the treatment of rheumatoid arthritis (RA). Therefore,...

Nonclinical safety of mavrilimumab, an anti-GMCSF receptor alpha monoclonal antibody, in cynomolgus monkeys: Relevance for human safety

Mavrilimumab (CAM-3001) is an investigational human IgG4 monoclonal antibody (MAb) targeting GM-CSF receptor alpha which is currently being developed for the treatment of RA. GM-CSF plays a central role in the pathogenesis of rheumatoid arthritis (RA) through the activation, differentiation, and survival of macrophages and neutrophils. To support clinical development, the nonclinical safety of mavrilimumab was evaluated in several studies with cynomolgus monkeys as the pharmacologically relevant species. Comprehensive toxicity parameters were assessed in each study, and treatment duration ranged from 4 to 26weeks. Mavrilimumab has an acceptable safety profile in monkeys with no changes in any parameters other than microscopic findings in lung. In several studies, minimal accumulation of foamy alveolar macrophages was observed. This finding was only seen in studies of at least 11weeks duration, was reversible following a dose-free recovery period and was considered non-adverse. At higher dose levels (≥30mg/kg/week), in a 26-week repeat-IV dose study, the presence of lung foreign material, cholesterol clefts, and granulomatous inflammation was also observed in a few animals and was considered adverse. The dose- and time-related accumulation of foamy macrophages in lung following exposure to mavrilimumab observed in several NHP studies was expected based upon the known role of GM-CSFRα signaling in the function of alveolar macrophages. Overall, a clean no-observed-adverse-effect-level (NOAEL) without any effects in lung was established and provided adequate clinical safety margins. In clinical studies in RA patients, mavrilimumab has demonstrated good clinical activity with adequate safety to support further clinical development. A Phase 2b study of mavrilimumab in subjects with RA is in progress.

SAT0260 Biomarkers Associated with Rheumatoid Arthritis Disease Activity Including Joint Damage Correlate with Changes in Clinical Response in Subjects Treated with Mavrilimumab at Doses above 10 Mg

BackgroundMavrilimumab is a human monoclonal antibody targeting the alpha subunit of GM-CSF receptor and was recently evaluated in RA subjects in a phase 2a study (EARTH). Results reported from the...

Identification of Immune Factors Regulating Antitumor Immunity Using Polymeric Vaccines with Multiple Adjuvants

The innate cellular and molecular components required to mediate effective vaccination against weak tumor-associated antigens remain unclear. In this study, we used polymeric cancer vaccines incorporating different classes of adjuvants to induce tumor protection, to identify dendritic cell (DC) subsets and cytokines critical to this efficacy. Three-dimensional, porous polymer matrices loaded with tumor lysates and presenting distinct combinations of granulocyte macrophage colony-stimulating factor (GM-CSF) and various Toll-like receptor (TLR) agonists affected 70% to 90% prophylactic tumor protection in B16-F10 melanoma models. In aggressive, therapeutic B16 models, the vaccine systems incorporating GM-CSF in combination with P(I:C) or CpG-ODN induced the complete regression of solid tumors (≤40 mm(2)), resulting in 33% long-term survival. Regression analysis revealed that the numbers of vaccine-resident CD8(+) DCs, plasmacytoid DCs (pDC), along with local interleukin (IL)-12, and granulocyte colony-stimulating factor (G-CSF) concentrations correlated strongly to vaccine efficacy regardless of adjuvant type. Furthermore, vaccine studies in Batf3(-/-) mice revealed that CD8(+) DCs are required to affect tumor protection, as vaccines in these mice were deficient in cytotoxic T lymphocytes priming and IL-12 induction in comparison with wild-type. These studies broadly demonstrate that three-dimensional polymeric vaccines provide a potent platform for prophylactic and therapeutic protection, and can be used as a tool to identify critical components of a desired immune response. Specifically, these results suggest that CD8(+) DCs, pDCs, IL-12, and G-CSF play important roles in priming effective antitumor responses with these vaccines.

Mavrilimumab: an evidence based review of its potential in the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) management has greatly improved with the development of biologic disease modifying antirheumatic drugs, but a proportion of patients do not improve despite the biologic drugs currently available. We need new biologic agents with novel mechanisms of action for the treatment of refractory patients. Recent evidence has shown that granulocyte-macrophage colony-stimulating factor (GM-CSF) is involved in the pathogenesis of RA. GM-CSF can exacerbate RA and elevated levels of this cytokine have been observed in synovial fluid from RA patients. Antagonism of GM-CSF can strikingly reduce established disease in mouse models of arthritis. Mavrilimumab, a human monoclonal antibody to GM-CSF receptor α, is a competitive antagonist of GM-CSF signaling. Phase I and II studies have shown good clinical response with a good safety profile in patients with mild to moderate RA, suggesting encouraging effects of mavrilimumab for the treatment of RA. This paper reviews the preclinical and clinical data evaluating the safety, tolerability, and efficacy of mavrilimumab in the treatment of RA.

Mechanisms of human eosinophil migration induced by the combination of IL-5 and the endocannabinoid 2-arachidonoyl-glycerol

Mechanisms of human eosinophil migration induced by the combination of IL-5 and the endocannabinoid 2-arachidonoyl-glycerol

In vitro efficacy of MAGMAS inhibition in prostate cancer.

281 Background: Prostate cancer is the second most common cancer worldwide in males. The initial treatment in advanced cases is medical or surgical castration. The outlook declines when prostate cancer advances independently, despite the aforementioned castration. Within the last ten years, a handful of new agents have proven effective in this castration-resistant phase, but finding more effective, novel ways of treating advanced prostate cancer is warranted. MAGMAS (mitochondria-associated, granulocyte-macrophage colony stimulating factor (GM-CSF) signaling molecule) is a protein ubiquitously expressed in eukaryotic cells that plays a key role in embryonal development in a variety of species. Overexpression of MAGMAS has anti-apoptotic effects, as GM-CSF is a growth factor essential for survival, proliferation and differentiation of cells. MAGMAS and GM-CSF receptor levels have been shown to be overexpressed in prostate cancer, but do not correlate with pathological grade or clinical stage. The purpose of our study was to evaluate the efficacy of a MAGMAS inhibitor, synthesized by Dr Bhaskar Das, in androgen-dependent and androgen-independent prostate cancer cell lines, as well as in a normal prostate cell line as another control. Methods: The different cell lines were treated with MAGMAS inhibitor at various concentrations in vitro. For analysis, we used MTT Cell Proliferation assay at 24 and 48 hours, per manufacturer’s protocol. We tested MAGMAS inhibitor effect on apoptosis/necrosis, cell migration and microtubule destabilization as well. Results: After prostate cancer cell lines were treated with MAGMAS inhibitor in vitro, cell proliferation and migration decreased, apoptosis and necrosis were induced, and microtubules were destabilized, all showing more impressive results in the androgen-independent cells. MAGMAS inhibition did not affect cell proliferation in the normal prostate cells. Conclusions: In vitro studies show MAGMAS inhibition proves efficacious in both androgen-dependent and androgen-independent prostate cancer cell lines. This will be evaluated further in a xenograft mouse model.

Juvenile myelomonocytic leukaemia-associated mutation in Cbl promotes resistance to apoptosis via the Lyn-PI3K/AKT pathway

Juvenile myelomonocytic leukaemia (JMML) is an aggressive myeloproliferative neoplasm in children characterized by granulocyte macrophage colony-stimulating factor (GM-CSF) hypersensitivity and resistance to chemotherapy. We recently identified c-Cbl (henceforth referred to as Cbl) as a GM-CSF receptor (GMR) responsive protein that targets Src for ubiquitin-mediated destruction upon GM-CSF stimulation and showed that a loss of negative regulation of Src is pivotal in the hyperactivation of GMR signalling in JMML cells. However, the mechanism regulating the chemoresistant nature of JMML has remained largely unknown. Here, we show that the JMML-associated Cbl mutant in complex with the Src family kinase Lyn promotes Cbl's adapter function, leading to increased association to PI3K regulatory subunit p85 and Lyn-dependent AKT pro-survival signalling. Notably, molecular or pharmacologic inhibition of the Lyn-PI3K/AKT pathway, but not the Ras/mitogen-activated protein kinase signalling axis, markedly increased the sensitivity of the otherwise chemoresistant Cbl mutant-JMML cells to chemotherapeutic agents currently used in the treatment of JMML patients. These results support the potential translational benefit of combining modalities that inhibit Lyn-PI3K/AKT signalling with traditional antileukaemia agents in the management of JMML.

Reduced BMPR2 expression induces GM-CSF translation and macrophage recruitment in humans and mice to exacerbate pulmonary hypertension

Idiopathic pulmonary arterial hypertension (PAH [IPAH]) is an insidious and potentially fatal disease linked to a mutation or reduced expression of bone morphogenetic protein receptor 2 (BMPR2). Because intravascular inflammatory cells are recruited in IPAH pathogenesis, we hypothesized that reduced BMPR2 enhances production of the potent chemokine granulocyte macrophage colony-stimulating factor (GM-CSF) in response to an inflammatory perturbation. When human pulmonary artery (PA) endothelial cells deficient in BMPR2 were stimulated with tumor necrosis factor (TNF), a twofold increase in GM-CSF was observed and related to enhanced messenger RNA (mRNA) translation. The mechanism was associated with disruption of stress granule formation. Specifically, loss of BMPR2 induced prolonged phospho-p38 mitogen-activated protein kinase (MAPK) in response to TNF, and this increased GADD34-PP1 phosphatase activity, dephosphorylating eukaryotic translation initiation factor (eIF2α), and derepressing GM-CSF mRNA translation. Lungs from IPAH patients versus unused donor controls revealed heightened PA expression of GM-CSF co-distributing with increased TNF and expanded populations of hematopoietic and endothelial GM-CSF receptor α (GM-CSFRα)-positive cells. Moreover, a 3-wk infusion of GM-CSF in mice increased hypoxia-induced PAH, in association with increased perivascular macrophages and muscularized distal arteries, whereas blockade of GM-CSF repressed these features. Thus, reduced BMPR2 can subvert a stress granule response, heighten GM-CSF mRNA translation, increase inflammatory cell recruitment, and exacerbate PAH.

EPA-0819 - Association of csfr2a gene rs4129148 and il3ra gene rs6603272 variants with schizophrenia

Schizophrenia is a devistating disorder which has a genetic component. Recently three reports showed that the IL3 gene, CSF2RA gene are associated with schizophrenia. The objective of the study was to show whether there was an association between the IL3R gene rs6603272 variant and the CSF2RA rs4129148 variants with schizophrenia. There was 264 schizophrenia patients and 222 controls from Romania. We utilized a PCR-RFLP method to genotype the subjects. The genotype frequencies of the IL3RA rs6603272 variant were TT, 54.2; TG, 39.8; and GG,6.1 % in cases and TT, 52.7; TG, 41.9; and GG,5.4 % in controls. The T allele frequency in both controls and cases was 74%. The genotype frequencies of the CSF2RA gene rs4129148 variant were GG, 40.2;GC,51.1; and CC, 8.7 % in cases and GG,39.2; GC,45.5; and CC,15.3 % in controls. The G allele frequency was 66% in cases and 62% in controls. The IL3RA gene rs6603272 variant was not associated with schizophrenia(X2=0.271; P=0.873). However The CSF2RA gene rs4129148 variant CC genotype was associated with schizophrenia(X2=5.079; P=0.024). The G allele was agenetic risk factor for schizophrenia (X2=5.079; P=0.024; OR=1.895, 95%CI= 1.080- 3.326). The controls were in Hardy-Weinberg equilibrium. But cases were not.

PReS13-SPK-1586: Biological agents for the treatment of rheumatic diseases: present and future targets of biologic therapy

The last decade has seen the advent of a variety of novel biologic agents, which are now used widely across a range of the inflammatory rheumatic diseases. Normally introduced after the trial and failure of conventional disease modifying anti-rheumatic drugs (DMARDs) the biologic class of medicines has been a revolutionary in terms of the scope and depth of response now sought in treating rheumatic diseases. Those best characterized are TNFi, IL-6Ri, rituximab and abatacept, all of which bring about improvement in clinical signs and symptoms, and variously mitigate articular destruction and improve function. Application of drugs developed for other indications but now being tested in rheumatic diseases has raised interest in the possible value of agents such as ustekinumab (inhibiting IL-12/23), and several antibodies that block the biology of IL-17. The latter appear to exhibit especial benefits in the spondylo arthropathies and psoriatic variants of inflammatory arthropathy. Other studies targeting GM-CSF receptor have similarly shown benefit in early studies in rheumatoid arthritis. The long promised possibility that chemokines and their receptors would deliver benefit now has some tentative evidence for benefit. In this presentation I shall describe the successes and failures of recent biologic therapeutic studies in the rheumatic disease, and wider inflammatory arena, and from this draw inferences as to new emerging understanding of the molecular taxonomy of the inflammatory diseases.

Gene Correction of Human Induced Pluripotent Stem Cells Repairs the Cellular Phenotype in Pulmonary Alveolar Proteinosis

Hereditary pulmonary alveolar proteinosis (hPAP) caused by granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor α-chain (CSF2RA) deficiency is a rare, life-threatening lung disease characterized by accumulation of proteins and phospholipids in the alveolar spaces. The disease is caused by a functional insufficiency of alveolar macrophages, which require GM-CSF signaling for terminal differentiation and effective degradation of alveolar proteins and phospholipids. Therapeutic options are extremely limited, and the pathophysiology underlying the defective protein degradation in hPAP alveolar macrophages remains poorly understood. To further elucidate the cellular mechanisms underlying hPAP and evaluate novel therapeutic strategies, we here investigated the potential of hPAP patient-derived induced pluripotent stem cell (PAP-iPSCs) derived monocytes and macrophages. Patient-specific PAP-iPSCs were generated from CD34(+) bone marrow cells of a CSF2RA-deficient patient with PAP. We assessed pluripotency, chromosomal integrity, and genetic correction of established iPSC lines. On hematopoietic differentiation, genetically corrected or noncorrected monocytes and macrophages were investigated in GM-CSF-dependent assays. Although monocytes and macrophages differentiated from noncorrected PAP-iPSCs exhibited distinct defects in GM-CSF-dependent functions, such as perturbed CD11b activation, phagocytic activity, and STAT5 phosphorylation after GM-CSF exposure and lack of GM-CSF uptake, these defects were fully repaired on lentiviral gene transfer of a codon-optimized CSF2RA-cDNA. These data establish PAP-iPSC-derived monocytes and macrophages as a valid in vitro disease model of CSF2RA-deficient PAP, and introduce gene-corrected iPSC-derived monocytes and macrophages as a potential autologous cell source for innovative therapeutic strategies. Transplantation of such cells to patients with hPAP could serve as a paradigmatic proof for the potential of iPSC-derived cells in clinical gene therapy.

Use of Induced Pluripotent Stem Cells to Recapitulate Pulmonary Alveolar Proteinosis Pathogenesis

In patients with pulmonary alveolar proteinosis (PAP) syndrome, disruption of granulocyte/macrophage colony-stimulating factor (GM-CSF) signaling is associated with pathogenic surfactant accumulation from impaired clearance in alveolar macrophages. The aim of this study was to overcome these barriers by using monocyte-derived induced pluripotent stem (iPS) cells to recapitulate disease-specific and normal macrophages. We created iPS cells from two children with hereditary PAP (hPAP) caused by recessive CSF2RA(R217X) mutations and three normal people, differentiated them into macrophages (hPAP-iPS-Mφs and NL-iPS-Mφs, respectively), and evaluated macrophage functions with and without gene-correction to restore GM-CSF signaling in hPAP-iPS-Mφs. Both hPAP and normal iPS cells had human embryonic stem cell-like morphology, expressed pluripotency markers, formed teratomas in vivo, had a normal karyotype, retained and expressed mutant or normal CSF2RA genes, respectively, and could be differentiated into macrophages with the typical morphology and phenotypic markers. Compared with normal, hPAP-iPS-Mφs had impaired GM-CSF receptor signaling and reduced GM-CSF-dependent gene expression, GM-CSF- but not M-CSF-dependent cell proliferation, surfactant clearance, and proinflammatory cytokine secretion. Restoration of GM-CSF receptor signaling corrected the surfactant clearance abnormality in hPAP-iPS-Mφs. We used patient-specific iPS cells to accurately reproduce the molecular and cellular defects of alveolar macrophages that drive the pathogenesis of PAP in more than 90% of patients. These results demonstrate the critical role of GM-CSF signaling in surfactant homeostasis and PAP pathogenesis in humans and have therapeutic implications for hPAP.

RUNX1-ETO Expressing Hematopoietic Stem/Progenitor Cells (HSPCs) Display a Hypersensitivity To GM-CSF That Reduces Leukemogenic Potential

RUNX1-ETO (also known as AML1-ETO and AML1-MTG8) is a fusion gene generated from t(8;21), which is a common chromosome translocation in acute myeloid leukemia (AML). It has been shown that t(8;21) requires additional aberrations to induce leukemia. Interestingly, 32-59% of t(8;21) patients also display loss of a sex chromosome (LOS) in their leukemia cells. Therefore, loss of the genes located on the sex chromosomes, especially in the pseudoautosomal regions (PARs) that are shared between the X and Y chromosomes, may contribute to RUNX1-ETO leukemia development. One gene of interest in the PARs is CSF2RA, which encodes the alpha subunit of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor. When the GM-CSF receptor is bound to its ligand, downstream signaling events promote various functional outcomes including proliferation, differentiation, self-renewal, and survival of myeloid cells. Thus, GM-CSF signaling has the potential to regulate both normal and malignant hematopoiesis. We previously reported that mice expressing RUNX1-ETO in GM-CSF deficient hematopoietic cells displayed higher incidence of leukemia (Matsuura S et al. 2012 Blood 119:3155). This result suggests that GM-CSF signaling is inhibitory to RUNX1-ETO dependent leukemogenesis. Furthermore, GM-CSF treatment reduces the self-renewal potential of RUNX1-ETO expressing cells and promotes myeloid differentiation in replating assays. We therefore hypothesize that the negative effect of GM-CSF on RUNX1-ETO induced leukemia development is due to the activation of selected GM-CSF downstream signaling pathway(s) that diminish self-renewal capacity and promote myeloid differentiation.To understand the molecular mechanism of the negative effect of GM-CSF on t(8;21) leukemogenesis, in the current report, we conducted a gene expression profiling assay to examine the effect of GM-CSF on RUNX1-ETO cells. MigR1 vector control or MigR1-RUNX1-ETO retrovirus transduced lineage negative/c-Kit positive (Lin-/c-Kit+) murine hematopoietic stem/progenitor cells (HSPCs) were cultured with or without GM-CSF for 24 hours. Then, Lin-/c-Kit+/GFP+ HSPCs were isolated for the profiling study. We observed little response to GM-CSF in control HSPCs, with only 4 genes being differentially expressed after a 2-fold cutoff. Conversely, 122 genes were differentially expressed in RUNX1-ETO cells treated with GM-CSF. These results clearly indicate that RUNX1-ETO specifically enhances GM-CSF responsiveness in HSPCs. Gene Set Enrichment Analysis (GSEA) of the differentially expressed genes in RUNX1-ETO cells reveals that this response resembles that of GM-CSF-induced myeloid differentiation. Furthermore, pathway analysis of these differentially expressed genes predicts MEK1/2 and ERK1/2 to be activated after GM-CSF treatment in RE cells. We previously reported that ERK1/2, downstream targets of MEK1/2, are hyper-phosphorylated after GM-CSF treatment of RUNX1-ETO cells, and MEK-ERK activation has been shown to regulate cell proliferation and myelopoiesis. Other GM-CSF induced genes are predicted targets of MYD88. MYD88 is upregulated during myeloid differentiation. Its in vivo knockout has been reported to result in an increase of hematopoietic stem cells (HSCs) and reduction of mature granulocytes. Most interestingly, a subset of genes upregulated in GM-CSF treated RUNX1-ETO cells are predicted to be activated by CEBPβ. CEBPβ can heterodimerize with CEBPα and is induced during myelopoiesis, critical for macrophage differentiation, capable of promoting granulopoiesis, and involved in regulating granulopoiesis in vivo.In conclusion, our data suggest that RUNX1-ETO expression results in hyper-responsiveness to GM-CSF. Such enhanced GM-CSF signaling activates the expression of a specific group of genes and results in the reduced self-renewal capacity and increased myeloid differentiation of HSPCs. These GM-CSF effects are likely involved in reducing the leukemogenic potential of RUNX1-ETO and may be considered for specific therapeutic interventions. Disclosures:No relevant conflicts of interest to declare.